In case of a submersible pump used for civil engineering works, a pump exhibiting high resistance against abrasion is usually required since liquid handled by such pump contains a substantial amount of earth and sand. Especially, when a connected pipe is bent due to its flexibility or when a delivery pipe is led to a high position and the level of liquid is relatively low so that air is likely to be introduced into the pump, the earth and sand introduced into the pump may whirl within the pump but may not be easily discharged outwardly from the pump. If the pump is operated for a long period under such conditions, the inner surfaces of the pump will be subjected to abrasion by the earth and sand. For instance, in the conventional pump comprising a motor section, a pump section and an intermediate casing between the two sections, the pump section including a semi-open type impeller, the surface of the intermediate casing facing the backside of the main plate of the impeller and the inner surface of the pump casing wall having a suction opening is subjected to severe abrasion. Also, the intermediate casing is usually secured to the motor casing of the motor section in such pump by plural bolts whose heads are exposed to the impeller chamber of the pump casing and, thus, the heads of the bolts are subjected to abrasion to become unrotatable by wear thereby causing the pump to be incapable of being disassembled.
If such pump is arranged to be a vortex pump wherein a vortex chamber is provided between an open end side of the semi-open type impeller and the end wall of the pump casing having the suction opening, wear on the inner surface of the end wall would be remarkably reduced since the flowing speed of the liquid near the inner surface of the end wall is reduced due to the presence of the space between the open end of the impeller and the inner surface of the end wall. However, the wear due to abrasion of the bolt heads and the surface of the intermediate casing may not be effectively prevented by the mere presence of the vortex chamber.